Need a little help on the behavior of an L293D h-bridge. It's to be used for a small robot. 2 channel variable PWM provided by a PIC12F509 in increments of 1% duty cycle @ 200HZ.

On the L293D when i apply 5vdc to Pin 2 (input-1) i get full voltage at Pin 3 (motor-1). This happens regardless of the state of the PWM input Pin 1 (Inhibit-1).

I have even physically removed the PIC from the circuit.

Could i have cooked L293D?

Any thoughts?

Thanx,
Zeb

 

may be is fryed, but you can figure it out testing it , now, max. input voltage is 7 volts, I guess you did not over pass that? now, what did you connect to the output? you have 600 mili amps... and the enable is high right? I mean a secure high level not disconnected, you can connect it to 5 volts direct or to the enabling source...
try the table with 5 volts and find if is dead or not...
in enable out
H H H
L H L
H L Z
L L Z

 

The absolute max peak output current of an L293D is only 1.2A. The motor draws a very high current when it starts and when it is stalled. The over-current quickly melts an output transistor so it is on all the time.

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Uncle $crooge

 

what about the input from the pic? are you sure it is 200 hz pulses , did you see it whit oscilloscope or scope meter? or measure it with meter?

 

All,

Thanks for the thoughts and replies!!!

As it turns out a little thoughtfull time spent with an O-scope / DVM, (did i mention the cold beer) and who can tell what might happen.

Two (on each side) 10k pulldown resistors fixed the problem.

I tied input-1 and input-2 to gnd with 10k pulldowns

Applied the PWM signal to Chip-Inhibit-1

no voltage on motor output

Applied +5 to input-1 and the appropriate voltage level apeared on the motor output

Decreased duty cycle to 0% and voltage followed for both fwd and reverse polarity on the motor output

Moral of the story "No floating PINS"

Thanks for the help!!

Regards,
Zeb

 

Great, yes, high impedance give you that kind of problem, as I said before, a secure level, 0 or 1 , but secure...not high Z...

 

it is fried, search no more. i had the exact same problem.

those litle motors in toy cars tend to drain a lot more current that you would execpt a motor of this size would.... i overcame this by providing very good heatsinking and ventilation..

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Ibrahim Kamal
check my electronics and robotics page: http://www.ikalogic.com/

 

Are you talking about the continious current?

I haven't looked at the datasheet but it's highly likely it can withstant a surge of at least 10 times that value for 50mS; it depends on the safe operating area of the transistor.

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The L293D is made for driving tiny low power motors.
Its max continuous current is 600mA.
Its max peak current (for only 100us non-repetitive) is only 1.2A.

__________________
Uncle $crooge

 

That's pretty poor, I can't think of any motors that won't take any less than a 5A surge whewn starting up. What use is an IC that can only take 1.2A?

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez
And http://www.silicontronics.com, same screen name as here.

 

Hero, 50+% of the motors I own aren't capable of drawing 1.2AMPS stalled. Several of them would actually burst into flames if that much current was going through them even for a few ms's

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